Cargo ships carrying crude oil, etc are provided with ballast tanks to balance their body during navigation.
Usually, the ballast tanks are filled with ballast water when crude oil, etc are not on-board, while the ballast water is discharged before crude oil, etc are loaded.
Ballast water is necessary for safe navigation of ships, and it is usually taken from sea water in the port where cargo handling is undertaken. The total quantity of ballast water used in the world is estimated at 3 to 4 billion tons a year.
Ballast water contains aquatic organisms which inhabit in ports where the ballast water is drawn, and the aquatic organisms are conveyed to other countries as the ships move. Consequently, destruction of ecological system is increasingly serious that alien organism species take the place of indigenous species in the sea area.
As seriously considered the above background, a diplomatic conference at the International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments to make the obligation of implementing ballast water control by use of any ballast water treatment apparatus be applied to ships to be built from 2009 onward.
In addition, the convention prescribed the ballast water discharging standard as shown in Table 1:
TABLE 1Ballast WaterItemsQuality CriteriaSizeAquatic Organisms10 unit/ml10-50 μmAquatic Organisms10 unit/m350 μm or moreIndicatorEscherichia Coli250 cfu/100 ml/MicrobesVibrio cholerae1 cfu/100 ml/(O1 and O139)Genus Enterococcus100 cfu/100 ml/
Accordingly, it is now a matter of great urgency to develop a sterilization and/or elimination technology in the ballast water which can solve the above problem.
Conventionally, a technology for sterilization by means of injecting ozone into ballast water in parallel with injecting steam and further generating micro bubbles of ozone to promote formation of hydoxyradicals to reduce consumption of ozone has been offered, as seen in Unexamined Patent Application Publication No. 2004-160437(JP).